Miniaturization of high frequency power converters

DC-DC power converters are major contributors to volume and weight of the most of electronic systems. With the revolutionary development in electronic equipment and added functionality, power converters are required to keep pace with such developments. Therefore, the need for smaller, lighter and more reliable power supply units (PSUs) became a driving force for researchers.

To develop such power converters, new semiconductor devices, semiconductor materials, innovative packaging, medium and high frequency (300 KHz – 30 MHz) magnetics, in addition to the development of converter topologies form the pillars for achieving efficient and high-density power conversion. This study represents an investigation of the technological pathways for achieving various levels of integration of power converters.

The three major contributions carried out throughout this work are summarized as follows. First, a study of two semiconductor technologies, which are used in the majority of modern integrated power supplies, is conducted. These two technologies are silicon transistors and gallium nitride transistors.

For the silicon semiconductor-based devices, a 140 V power MOSFET with a custom layout is manufactured in a 0.18 µm commercial silicon-on-insulator (SOI) process. The device resistance and capacitances are characterized using a custom designed small signal characterization setup with high voltage bias capabilities.

A 32 V class-E resonant boost converter with a self-oscillating gate driver is designed around the MOSFET to test its switching performance at very high switching frequency. The converter is tested at 30 MHz and achieved a power density of 2 W/cm3. Second, Gallium nitride FETs are utilized in an 80 V module as well as in a power supply in package (PSiP) achieving a power density of 20 W/cm3 and an area density of 9.4 W/cm2 .

Both versions of the prototypes are tested in the frequency range between 2 MHz and 12 MHz. Aircore solenoid inductors were used in the implementation. The PSiP version of the converter shows a significant enhancement over the module version thanks to the advanced thermally enhanced molding. Third, Energy storing elements - specifically, inductors and capacitors - are studied aiming for higher density power supplies.

Air-core magnetics and printed circuit board spiral inductors are studied. Circular spiral and square spiral inductor structures are implemented and tested in a 10 MHz switch mode power supply. Custom-made Micro-Electro-Mechanical Systems (MEMs) fabricated toroidal inductors in silicon substrates were used to design an 8 V PSiP built on a 3D magnetic-silicon-interposer achieving a power density of 36 W/cm3 .

This converter integrates a 3D toroidal inductor in silicon, gallium nitride FETs, gate drivers, input capacitors, and output capacitors, forming a buck converter in 8 mm x 4.5 mm x 1.2 mm volume with a clear path to reduce the profile to 0.6 mm. The converter has a switching frequency of 21 MHz and achieves 83% efficiency.

This converter represents an intermediate step for realizing a fully integrated power supply on chip (PwrSoC). In conclusion, this study represents an exploration of the potential technologies, topologies and methods which have high potential for integration and miniaturization. The study covers topologies including soft-switching buck converters, Class-E resonant converters, and ClassDE resonant converters.

Integrated circuit design technologies including silicon-in-insulator and high-voltage CMOS technologies are used to design two chips. Packaging technologies like module integration, power-supply-in-package, and thermally enhanced epoxy molding are used, not only to achieve tighter integration for the internal components, but also to enhance the thermal and mechanical performance of the prototypes.

Finally, control techniques like on-off control, frequency control, and phase-shift control were investigated to achieve different levels of control for the output voltage or the output current of the power converters. Adaptive dead-time control is identified as one of the most important circuits to integrate in case of applying continuous operation control methods, like pulse width modulation control, frequency control, or phase shift control, as it is a must to use in combination with soft-switching techniques.

With the proper application of the knowledge gained throughout this work, it is possible to achieve high power density converters, and potentially realize the vision of a fully integrated power-supply-on-chip (PwrSoC).